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Aviation Investigation Report A98H0003

The occurrence aircraft was equipped with a digital FDR and a CVR. The FDR was an L3 Communications (Loral/Fairchild) model F-1000, which records about 250 parameters in solid state memory. (STI1-56) The recorder contained about 70 hours of continuous flight data, which included the accident flight, and the six previous flights. The FDR, as configured, did not record the parameters "Lavatory Smoke" and "Cabin (Cargo) Smoke." Nor did the FDR record any parameters related to the IFEN system. The data recorded on the FDR was of good technical quality.

The CVR was an L3 Communications (Loral/Fairchild) model 93-A100-81. (STI1-57) The recording medium was 1/4-inch tape on a continuous loop. The design provided for a nominal recording time of 30 minutes. The actual length of the CVR recording was 32 minutes, 24 seconds, starting at 0053:17 and stopping at 0125:41. The CVR recorded on four separate tracks: the output of each of the two pilot's audio management units (AMU); the cabin interphone or public address audio, whichever is selected; and the cockpit area microphone (CAM).

The CVR-recorded audio was of fair technical quality overall. Prior to when the pilots donned their oxygen masks, which incorporate a "hot" microphone[48] input to the pilot and co-pilot channels of the CVR, cockpit conversations were recorded only on the CAM channel. While in cruise flight, the pilots were not using their headsets, which incorporate integral boom microphones that provide better quality CVR recording than does the CAM. The industry norm is to not use the headsets while cruising at high altitude, and there was no regulation or company policy requiring them to do so. Despite extensive filtering attempts, some of the audio information recorded by the CAM on the CVR was difficult or impossible to decipher because of masking, either by the ambient cockpit noise, or by background ATS radio communications emanating from the cockpit speaker. The pilots' internal verbal communication was mostly in the SwissGerman language.

[48] A hot microphone permits conversations to be heard continuously between the flight crew positions without any requirement to select an intercom switch.